Membrane traffic is required for a broad range of essential cellular functions, such as controlling the accessibility of cell surface receptors, the translocation of glucose transporters in response to insulin, antigen presentation, neuronal transmission and the establishment and maintenance of epithelial cell polarity. Therefore, the regulation of membrane traffic is directly relevant to a broad range of human diseases including cancer, diabetes and neural degeneration. Rab GTPases are key regulators of membrane traffic. By recruiting and activating a functionally diverse set of effectors, a single Rab GTPase can coordinate the various sub- reactions within a given stage of membrane traffic, including vesicle budding, delivery, tethering and fusion. Furthermore, our results indicate that adjacent stages of transport can also be coupled through coordinated rab regulation. We recently defined a rab guanine nucleotide exchange factor (GEF) cascade in which one rab, in its GTP-bound state, recruits the GEF that activates the next rab along the exocytic pathway. We also have preliminary evidence for a rab GTPase activating protein (GAP) cascade operating in a counter current fashion. Here the downstream rab recruits the GAP that inactivates the upstream rab. The net effect is rab conversion in which a given patch of membrane starts out labeled with one rab, but over time becomes labeled with another rab. Since each rab recruits and activates a distinct set of effectors, this leads to a functional maturation of the membrane. We will explore these two cascade mechanisms in further detail and test the physiological consequences of uncoupling adjacent stages of membrane traffic. We will test the role of a Sec4p effector in SNARE assembly and explore the roles of several new putative Sec4p effectors. Through these studies we will begin to define the exocytic pathway as a fully coordinated system, rather than as a collection of isolated sub-reactions. Membrane traffic is the mechanism by which material is transferred between different compartments within the cell and the regulation of membrane traffic is directly relevant to a broad range of human diseases including cancer, diabetes and neural degeneration. This study addresses the molecular mechanisms by which different stages of membrane traffic are coordinately regulated. [unreadable] [unreadable] [unreadable]